MECHANISMS OF METHANOL ADSORPTION ON SILICALITE AND SILICA - IR-SPECTRA AND AB-INITIO CALCULATIONS

Combined IR and ab-initio studies suggest the presence of two chemisor bed species of methanol on ZSM5-like silicalite, namely, free and hydr ogen-bonded -->SiOCH3 groups, which are in thermodynamic equilibrium. Upon decreasing the temperature, the relative concentration of the fre e -->SiOH groups increases due to the -->SiOH transition from the hydr ogen-bonded -->SiOCH3 groups into hydrogen-bonded -->SiOH aggregates. Physically adsorbed methanol forms large hydrogen-bonded polymers, eve n at high concentration of unoccupied -->SiOH groups. The CH frequenci es of the chemisorbed and physically adsorbed species on silicalite ar e equal to those on silica, indicating the identity of these forms on both solids. Owing to the extremely high local concentration of -->SiO H groups in atomic-scale defects in silicalite, the effect of the reve rsible transformation of two chemisorbed forms to each other, on chang ing the temperature, is more pronounced for this solid than for silica .